Nano-particulate tin coated products

ABSTRACT

A process for the preparation of an inorganic filler material coated with a nano-particulate tin compound which comprises forming a slurry of the filler material in an aqueous colloidal suspension of a tin compound, whereby the tin compound is directly precipitated from the colloidal suspension onto the surface of the filler, separating the inorganic filler from the colloidal suspension and, optionally, heating the coated filler to convert the hydrated tin compound to the anhydrous form. A fire retardant material comprises a particulate inorganic material coated with a nano-particulate tin product in an amount of from 1 to 100% by weight, based on the weight of the inorganic filler.

The present invention relates to the preparation of nano-particulatetin-containing species, their deposition on to particulate substrates,and the application of the resulting products as fire-retardant fillers,catalysts, ion exchange systems and electroconductive powders and films.

Tin(IV) oxide and related inorganic tin compounds are known as non-toxicflame retardants and smoke suppressants for a wide range of organicpolymers. For example, tin compounds such as zinc hydroxystannate (ZHS),zinc stannate (ZS) and tin(IV) oxide (SnO₂) have been shown to exhibitgood flame-retardant and smoke-suppressant properties inhalogen-containing polymer formulations such as PVC and polychloroprene,and in other plastics and rubbers, particularly those which containorganic chlorine or bromine compounds as additive-type flame retardants.The non-toxic nature of the inorganic tin compounds, combined with theirdual flame-retardant and smoke-suppressant activity, has generatedconsiderable interest in their use as alternatives to antimony trioxidein halogenated polymer systems.

It has been found that the conditions used to prepare these tincompounds can be modified in such a way that the resulting powderedproducts have greatly reduced particle sizes compared with powdersproduced by conventional methods. Furthermore, it has been shown thatsuch ‘ultrafine’ powders have improved flame retardancy and smokesuppression compared with standard powders, when incorporated intopolymeric substrates. Thus WO 90/09962 discloses methods for producingsub-micrometre scale ZHS and ZS powders, and their application asfire-retardant additives for organic polymers.

It has also been disclosed in EP-B1-0833862 that the use of fillerpowders comprising a particulate inorganic filler material, theparticles of which are coated with the ultrafine ZHS/ZS particlesdescribed in WO 90/09962, gives improved flame retardancy and smokesuppression as compared with the use of the fillers themselves, or withsimple mixtures of the two components (filler and tin compound). Lowerlevels of the coated fillers, compared with the corresponding uncoatedgrades, are required for a given fire-retardant performance, and thisreduction in filler loading leads to better polymer processing andimproved physical and mechanical properties.

Processes for preparing stable aqueous colloidal sols of tin(IV) oxideand related inorganic tin compounds, such as tin(IV) phosphate, tin(IV)borate and tin(IV) tartrate, have been disclosed in EP-B1-0896649. Thecolloidal products prepared using this process, and which containnanometre-scale particles, are used to impregnate cellulosic materials,such as paper, card, cardboard or pulp. These colloidal tin additivesrepresent an improvement in dispersion of another order of magnitudecompared with the ultrafine powders discussed above.

It has now been found, in accordance with the present invention, that itis possible to coat inorganic particulate substrates withnanometre-scale particles of tin compounds using, as the startingmaterial, the aqueous colloidal suspensions as described above.Accordingly, inorganic particulate fillers, such as alumina trilydrate(ATH), magnesium hydroxide, calcium carbonate, magnesium carbonate, zincborate, silica, talc, anhydrous alumina, sodium bentonite, nanoclays(such as those produced as the reaction product of a smectite-type clayand a mixture of a quaternary ammonium compound and a chain transferagent, or those produced from natural or synthetic layered silicatesbased on montmorillonite together with a suitable organic interactionagent usually a quaternary ammonium compound), and naturally occurringmixtures of magnesium calcium carbonate (Huntite) and hydrated basicmagnesium carbonate (Hydromagnesite), may be coated using such aprocess.

According to the present invention there is provided a process for thepreparation of an inorganic material which comprises a particulateinorganic filler selected from alumina trihydrate, magnesium hydroxide,calcium carbonate, magnesium carbonate, zinc borate, silica, talc,anhydrous alumina, sodium bentonite, nanoclays, magnesium calciumcarbonate, hydrated basic magnesium carbonate, or mixtures thereofcoated with nano-particulate tin compounds, which process comprises thesteps of:

-   -   (i) forming a slurry of the said particulate inorganic filler in        an aqueous colloidal suspension of a tin compound, whereby the        tin compound is directly precipitated from the colloidal        suspension onto the surface of the said filler,    -   (ii) separating the said inorganic filler from the colloidal        suspension, either with or without the adjustment of the pH of        the slurry prior to separation,    -   (iii) optionally, heating the filler coated with the        nano-particulate tin compound, in order to convert the hydrated        tin compound to an anhydrous form.

In step (i) of the process of the present invention the aqueouscolloidal suspension of the tin compound may contain other chemicalcompounds such as an inorganic or carboxylic acid, or a soluble metalsalt.

The inorganic or carboxylic acid which may optionally be used in step(i) of the process of the present invention may be phosphoric acid,boric acid, tartaric acid or citric acid. The soluble metal salt whichmay optionally be used in step (i) of the process of the presentinvention may be a salt of zinc, iron, cobalt, nickel or copper.

In step (ii) of the process of the present invention the pH of thecolloidal suspension may be adjusted by the addition of an acid, such asnitric acid, or a base such as ammonia.

The coated inorganic particulate filler produced by the presentinvention may find application where it is desired to apply a coating ofa tin compound having a high surface area on to the surface of aninorganic substrate. Thus, for example, the nano-particulate coatedfillers of the present invention may be used in applications such asfire-retardant fillers for polymers, heterogeneous catalysts, ionexchange systems and electroconductive powders and films.

The present invention also provides a polymer composition comprising apolymer and a fire-retardant coated inorganic particulate fillerproduced by the process as defined above. Suitably, the coatedparticulate inorganic fire-retardant filler will be present in thepolymer composition in an amount from 5 to 400% by weight, based on theweight of polymeric material, preferably 20 to 200% by weight on thesame basis. Further, the fire-retardant coated filler itself preferablycontains the nano-particulate tin compound in an amount from 1 to 100%,preferably 2 to 50% by weight, based on the weight of the inorganicfiller.

The inorganic fillers which may be used in the present invention includealumina trihydrate (ATH), magnesium hydroxide, calcium carbonate,magnesium carbonate, zinc borate, silica, talc, anhydrous alumina,sodium bentonite, nanoclays (such as those produced as the reactionproduct of a smectite-type clay and a mixture of a quaternary ammoniumcompound and a chain transfer agent, or those produced from natural orsynthetic layered silicates based on montmorillonite together with asuitable organic interaction agent usually a quaternary ammoniumcompound), and naturally occurring mixtures of magnesium calciumcarbonate (Huntite) and hydrated basic magnesium carbonate(Hydromagnesite). Suitably, the particles of the inorganic filler willhave an average particle size in the range of from 0.01 to 100micrometres, preferably from 0.1 to 20 micrometres.

The polymer component of the composition may be any of a wide variety ofmaterials, e.g. thermoplastic, thermosetting or elastomeric, and mayinclude halogen-containing and halogen-free formulations. The polymersmay be in the form of block, sheet, foam, fibre or in compounded form,e.g. in a paint or similar coating composition.

In order that the invention may be further understood, the followingExamples are given by way of illustration only.

EXAMPLE 1

Anhydrous tin(IV) chloride (86 g) was added cautiously to distilledwater (1 litre) and the resulting solution was adjusted to pH 9 usingaqueous ammonia (s.g. 0.88). The slurry was then adjusted to pH 4 usingdilute (1:1) nitric acid and the resulting white precipitate of hydroustin(IV) oxide was separated by centrifugation and washed with distilledwater until free of chloride ion (negative aqueous silver nitrate test).Aqueous ammonia (s.g. 0.88) was added to the washed precipitate until apH of approximately 10 was reached, following which the slurry washeated to 60° C. and maintained at this temperature for 2 hours, therebyeffecting its peptisation to form a clear colloidal sol of tin(IV)oxide. The sol was allowed to cool to room temperature and distilledwater was added to make up the volume of the colloid to 1 litre. 40 mlof the resulting colloid (pH 10.4) were added to magnesium hydroxide (5g) in a 50 ml screw-capped tube, and the tube was agitated on amechanical shaker for 3 hours. The resulting product was separated bycentrifugation, washed with distilled water, and dried in air at 105° C.for 2 hours, to give a white powder which analysed as 17.9% Sn byweight.

EXAMPLE 2

A ‘half-strength’ aqueous tin(IV) oxide colloid was prepared by adding500 ml of distilled water to 500 ml of the colloid produced inExample 1. 40 ml of the resulting colloid (pH 10.3) were added tomagnesium hydroxide (5 g) in a 50 ml screw-capped tube, and the tube wasagitated on a mechanical shaker for 3 hours. The resulting product wasseparated by centrifugation, washed with distilled water, and dried inair at 105° C. for 2 hours, to give a white powder which analysed as11.0% Sn by weight.

EXAMPLE 3

Anhydrous tin(IV) chloride (344 g) was added cautiously to distilledwater (4 litres) and the resulting solution was adjusted to pH 9 usingaqueous ammonia (s.g. 0.88). The slurry was then adjusted to pH 4 usingdilute (1:1) nitric acid and the resulting white precipitate of hydroustin(IV) oxide was separated by centrifugation and washed with distilledwater until free of chloride ion (negative aqueous silver nitrate test).Aqueous ammonia (s.g. 0.88) was added to the washed precipitate until apH of approximately 10 was reached, following which the slurry washeated to 60° C. and maintained at this temperature for 2 hours, therebyeffecting its peptisation to form a clear colloidal sol of tin(IV)oxide. The sol was allowed to cool to room temperature and distilledwater was added to make up the volume of the colloid to 4 litres.Magnesium hydroxide (500 g) was added to this colloid (pH 10.0) and theresulting slurry was stirred vigorously for 3 hours at room temperature.The resulting product was separated by centrifugation, washed withdistilled water, and dried in air at 105° C. for 16 hours, to give awhite powder which analysed as 19.5% Sn by weight.

EXAMPLE 4

A colloidal suspension of tin(IV) phosphate was prepared by addingphosphoric acid (81 g of 85% by weight aqueous H₃PO₄) with stirring to awarm (65° C.) 1 litre solution containing sodium hydroxystannate (80 g).40 ml of the resulting colloid (pH 3.8) were added to magnesiumhydroxide (5 g) in a 50 ml screw-capped tube, and the tube was agitatedon a mechanical shaker for 3 hours. The resulting product was separatedby centrifugation, washed with distilled water, and dried in air at 105°C. for 2 hours, to give a white powder which analysed as 15.3% Sn and7.0% P by weight.

EXAMPLE 5

A ‘half-strength’ aqueous tin(IV) phosphate colloid was prepared byadding 500 ml of distilled water to 500 ml of the colloid produced inExample 4. 40 ml of the resulting colloid (pH 4.0) were added tomagnesium hydroxide (5 g) in a 50 ml screw-capped tube, and the tube wasagitated on a mechanical shaker for 3 hours. The resulting product wasseparated by centrifugation, washed with distilled water, and dried inair at 105° C. for 2 hours, to give a white powder which analysed as8.2% Sn and 4.1% P by weight.

EXAMPLE 6

A colloidal suspension of tin(IV) borate was prepared by adding boricacid (93 g) with stirring to a warm (65° C.) 1 litre solution containingsodium hydroxystannate (26.7 g). 40 ml of the resulting colloid (pH 6.6)were added to magnesium hydroxide (5 g) in a 50 ml screw-capped tube,and the tube was agitated on a mechanical shaker for 3 hours. Theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 7.0% Sn and 0.4% B by weight.

EXAMPLE 7

A ‘half-strength’ aqueous tin(IV) borate colloid was prepared by adding500 ml of distilled water to 500 ml of the colloid produced in Example6. 40 ml of the resulting colloid (pH 7.2) were added to magnesiumhydroxide (5 g) in a 50 ml screw-capped tube, and the tube was agitatedon a mechanical shaker for 3 hours. The resulting product was separatedby centrifugation, washed with distilled water, and dried in air at 105°C. for 2 hours, to give a white powder which analysed as 3.3% Sn and0.1% B by weight.

EXAMPLE 8

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of a proprietary mineral filler, ‘Ultracarb’(ex-Microfine Minerals)—itself a mixture of Huntite (magnesium calciumcarbonate) and Hydromagnesite (hydrated basic magnesium carbonate), in a50 ml screw-capped tube. The tube was agitated on a mechanical shakerfor 3 hours and the resulting product was separated by centrifugation,washed with distilled water, and dried in air at 105° C. for 2 hours, togive a white powder which analysed as 18.8% Sn by weight.

EXAMPLE 9

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of calcium carbonate in a 50 ml screw-capped tube.The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 18.3% Sn by weight.

EXAMPLE 10

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of zinc borate in a 50 ml screw-capped tube. Thetube was agitated on a mechanical shaker for 3 hours and the resultingproduct was separated by centrifugation, washed with distilled water,and dried in air at 105° C. for 2 hours, to give a white powder whichanalysed as 22.7% Sn by weight.

EXAMPLE 11

40 ml of a colloidal suspension of tin(IV) phosphate, prepared as inExample 4, were added to 5 g of Ultracarb in a 50 ml screw-capped tube.The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 15.1% Sn and 5.9% P by weight.

EXAMPLE 12

40 ml of a colloidal suspension of tin(IV) phosphate, prepared as inExample 4, were added to 5 g of zinc borate—in a 50 ml screw-cappedtube. The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 18.4% Sn and 10.6% P by weight.

EXAMPLE 13

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of standard grade alumina trihydrate (MartinalOL-104, ex-Martinswerk) in a 50 ml screw-capped tube. The pH of theresulting suspension was adjusted from 10.0 to 3.9 using dilute nitricacid and the resulting product was separated by centrifugation, washedwith distilled water, and dried in air at 105° C. for 2 hours, to give awhite powder which analysed as 21.1% Sn by weight.

EXAMPLE 14

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of a thermally-stable grade alumina trihydrate(Martinal TS-601, ex-Martinswerk) in a 50 ml screw-capped tube. The pHof the resulting suspension was adjusted from 10.0 to 4.1 using dilutenitric acid and the resulting product was separated by centrifugation,washed with distilled water, and dried in air at 105° C. for 2 hours, togive a white powder which analysed as 20.6% Sn by weight.

EXAMPLE 15

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of silica in a 50 ml screw-capped tube. The pH ofthe resulting suspension was adjusted from 10.0 to 4.0 using dilutehydrochloric acid and the resulting product was separated bycentrifugation, washed with distilled water, and dried in air at 105° C.for 2 hours, to give a white powder which analysed as 21.3% Sn byweight.

EXAMPLE 16

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of sodium bentonite in a 50 ml screw-capped tube.The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 17.8% Sn by weight.

EXAMPLE 17

40 ml of a colloidal suspension of tin(IV) oxide, prepared as in Example1, were added to 5 g of a proprietary nanoclay, Cloisite 30B(ex-Southern Clay Products Inc), in a 50 ml screw-capped tube. The tubewas agitated on a mechanical shaker for 3 hours and the resultingproduct was separated by centrifugation, washed with distilled water,and dried in air at 105° C. for 2 hours, to give a white powder whichanalysed as 4.7% Sn by weight.

EXAMPLE 18

40 ml of a colloidal suspension of tin(IV) borate, prepared as inExample 6, were added to 5 g of sodium bentonite in a 50 ml screw-cappedtube. The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 10.6% Sn and 1.6% B by weight.

EXAMPLE 19

40 ml of a colloidal suspension of tin(IV) phosphate, prepared as inExample 4, were added to 5 g of sodium bentonite in a 50 ml screw-cappedtube. The tube was agitated on a mechanical shaker for 3 hours and theresulting product was separated by centrifugation, washed with distilledwater, and dried in air at 105° C. for 2 hours, to give a white powderwhich analysed as 15.4% Sn and 5.7% P by weight.

EXAMPLE 20

The fire-retardant properties of the novel nano-particulate coatedproducts were investigated as follows. The powdered product described inExample 3, having a composition equivalent to ca. 25% tin(IV) oxide:75%magnesium hydroxide by weight, was incorporated into a halogen-freeethylene-vinyl acetate (EVA) cable formulation by compounding on a 16 mmtwin-screw extruder operating at 155-165° C., followed by compressionmoulding at a pressure of 20 tonnes and a temperature of 160° C. for 15mins.

In order to assess whether the novel coated products exhibit anysignificant improvements in fire-retardant performance over conventionaladditives, EVA compositions containing Mg(OH)₂ itself or a simplemixture of SnO₂+Mg(OH)₂, in equivalent level amounts to those present inthe coated product, were also prepared.

Flame-retardant and smoke-suppressant properties of the EVA samples wereassessed using two standard fire test techniques. The Limiting OxygenIndex (LOI) test is an indicator of ease of combustion in anoxygen-nitrogen atmosphere through downward burning of a verticallymounted specimen. The test method is generally reproducible to anaccuracy of ±0.5%, giving a useful comparison of the relativeflammability of different materials. Higher LOI values represent betterflame retardancy. Measurements were undertaken in accordance with BS2782 (Part 1, Method 141).

The Cone Calorimeter uses a truncated conical heater element toirradiate test specimens at heat fluxes from 10-100 kW/m², therebysimulating a range of fire intensities. The technique has been shown toprovide data that correlate well with those from full-scale fire tests.Cone Calorimeter tests were carried out in duplicate, using a 50 kW/m²incident heat flux, following procedures recommended in BS 476 (Part15). The technique provides detailed information about ignitionbehaviour, heat release and smoke evolution during sustained combustionand parameters reported in this work are defined below:

Peak Rate of Heat Release, Peak RHR (kW/m²)—taken as the peak value ofthe heat release rate vs. time curve, and considered to be the variablethat best expresses the maximum intensity of a fire, indicating the rateand extent of fire spread.

Fire Performance Index, FPI (m²·s/kW)— defined as the ratio of ignitiontime to Peak RHR, this parameter relates to time to flashover (or thetime available for escape) in a full-scale fire situation. Higher valuesof FPI represent improved fire safety.

Smoke Parameter, SP (MW/kg)—defined as the product of the measuredspecific extinction area and Peak RHR, this parameter is indicative ofthe amount of smoke generated in a full-scale fire situation.

Fire test results for the EVA samples are given in Table 1. Sample* LOIPeak RHR FPI SP Control - no additives 20.2 1404 0.04 665 100 phr Mg(OH)₂ 23.2 538 0.13 148 100 phr SnO₂ − coated 25.3 340 0.16 141 Mg (OH)₂100 phr SnO₂ + Mg (OH)₂ 23.4 576 0.10 219 mixture*phr = parts per hundred of resin.

Significant improvements in flame retardancy and smoke suppression aregiven by the nano-particulate SnO₂-coated Mg(OH)₂ compared to equivalentlevels of either Mg(OH)₂ alone, or a simple mixture of SnO₂ withMg(OH)₂. This increased activity is believed to arise from the vastlysuperior dispersion of the active tin species in the compositioncontaining the coated product produced in Example 3.

1. A process for the preparation of an inorganic material whichcomprises a particulate inorganic filler selected from aluminatrihydrate, magnesium hydroxide, calcium carbonate, magnesium carbonate,zinc borate, silica, talc, anhydrous alumina, sodium bentonite,nanoclays, magnesium calcium carbonate, hydrated basic magnesiumcarbonate, or mixtures thereof coated with nano-particulate tincompounds, which process comprises the steps of: (i) forming a slurry ofthe said particulate inorganic filler in an aqueous colloidal suspensionof a tin compound, whereby the tin compound is directly precipitatedfrom the colloidal suspension onto the surface of the said filler, (ii)separating the said inorganic filler from the colloidal suspension,either with or without the adjustment of the pH of the slurry prior toseparation, (iii) optionally, heating the filler coated with thenano-particulate tin compound, in order to convert the hydrated tincompound to an anhydrous form.
 2. A process as claimed in claim 1wherein in step (i) an acid or a soluble metal salt is added to theslurry.
 3. A process as claimed in claim 2 wherein the acid isphosphoric acid, boric acid, tartaric acid or citric acid.
 4. A processas claimed in claim 2 wherein in step (i) the soluble metal salt is asalt of zinc, iron, cobalt, nickel or copper.
 5. A process as claimed inclaim 1 wherein in step (iii) the filler coated with thenano-particulate tin compound is heated to a temperature in the range offrom 150 to 400° C.
 6. A process as claimed in claim 1 wherein the saidfiller particles have an average particle size in the range of from 0.01to 100 micrometres.
 7. A process as claimed in claim 6 wherein the saidfiller particles have an average size in the range of from 0.1 to 20micrometres.
 8. A fire-retardant material comprising a particulateinorganic filler coated with a nano-particulate tin compound in anamount of from 1 to 100% by weight, based on the weight of inorganicfiller.
 9. A fire-retardant material as claimed in claim 8 in which thenano-particulate tin compound is present in an amount of from 2 to 50%by weight, based on the weight of inorganic filler.
 10. A fire-retardantmaterial as claimed in claim 8 obtained by the process as claimed inclaim
 1. 11. A polymer composition comprising a polymeric material and afire-retardant material as claimed in claim
 8. 12. A polymer compositionas claimed in claim 11 which comprises from 5 to 400% by weight, basedon the polymeric material, of the fire-retardant material.
 13. A polymercomposition as claimed in claim 12 which comprises from 20 to 200% byweight, based on the polymeric material, of the fire-retardant material.